<p>Police are trying to find the assailant who gunned down a 27-year-old man as he rode a bicycle in Grant Park early Tuesday. </p><p>Officers responded at about 12:15 a.m. to the 300 block of East Jackson Drive for a call of a person shot, Chicago police said.</p><p>The bicyclist was riding on the sidewalk when he was shot in the upper leg, police said.